The present invention relates to an electrophotographic color copying paper and a copying method making use of this copying paper.
A typical conventional electrophotographic color copying technique employs a photosensitive device provided with a photoconductive layer which has a panchromatic photosensitivity. In the use of this photosensitive device, a color copy is produced by a process which includes four repetitional operations each having, after a uniform charging, a color light image exposure (B,G,R,Blk) through a color separation filter to obtain a latent image which is then devloped by a toner (yellow, cyanide, magenta, black) of each of colors corresponding to the colors of the color separation filter.
This conventional method, however, requires a complicated and time-consuming process and, in addition, the cost of the copying apparatus is raised and the size of the apparatus is increased impractically due to the necessity for the four developing units for different toner colors which occupy a large space.
To obviate this problem, Japanese Patent Publication No. 36019/1977 discloses a color copying paper having a conductive layer, a photoconductive layer on the conductive layer, and a color mosaic filter layer on the photoconductive layer. The color mosaic filter serves also as an insulating layer. In the use of this color copying paper, a color light image exposure is effected by an image-forming process employing a step in which a charging and a light image exposure are conducted simultaneously so as to form an electrostatic latent image on the color mosaic filter layer. This latent image is then developed by toners thus producing a color copy.
This color copying paper is superior in that it can produce a color copy by a single light image exposure, but still suffers from the following problems. Namely, since the color mosaic filter layer is formed in direct contact with the photosensitive layer, the property of the photoconductive layer is liable to be impaired during the production. In addition, since the color mosaic filter layer itself has to be charged, the filter layer itself has to be electrically insulating, which undesirably limits the material of the filter layer and the method of mixing of color elements. In order to form a sufficiently strong latent image on the filter layer, it is necesary to strictly control the thickness of this layer. For instance, an optical dimming of the copy image will be caused if the filter layer is made too thick.
For obtaining a sufficiently strong latent image, it is necessary to apply a high voltage to the color copying paper. A too high voltage, however, may cause a breakdown of the mosaic filter layer. It is to be noted also that a high voltage applied to the copying paper may cause a fading of colors in the color mosaic filter due to an ozone oxidation caused by a corona charging, resulting in a change in the color absorption characteristics to adversely affect the quality of the copy. For these reasons, it is not allowed to apply a sufficiently high voltage.
When a simpler image-forming process known as "Carlson Process" is used, the color mosaic filter layer serves as a barrier layer, so that electric charge is allowed to remain in the bright portion of the electrostatic latent image to cause a fog. The Carlson process, therefore, cannot be applied practically. It is, therefore, necessary to use the image-forming process having a step in which the charging and the light image exposure are conducted simultaneously, requiring a greater number of chargers.
In addition, since the toner is deposited to the color mosaic filter layer itself and fixed to the same by heat or pressure, the property of the filter is impaired in this fixing step to adversely affect the quality of the copy image.
It is to be noted also that a complicated process is required for the production of the color mosaic filter layer so that the cost of this layer is considerably high. Namely, in order that the mosaic filter layer has an insulating power suitable for serving as a part of the photosensitive device, and in order to avoid the undesirable dimming of the copy image, the thickness of this layer has to be controlled strictly to range between 20 and 80.mu.. When the filter layer is made of a plastic, the thickness control is conducted through a number of steps including heating of the plastic up to a temperature above the room temperature and rolling the same to reduce the thickness, resulting in a rise of the production cost. This problem is serious particularly when the color copying paper is intended for use as expendable supplies as, for example, copying paper for CPC (Coated paper Copier).